Oncolytic Viruses
Oncolytic virotherapy is an emerging treatment platform for cancer therapy. Oncolytic viruses are viruses that selectively replicate in cancer cells that possess specific oncogenic phenotypes, thereby killing cancer cells whilst sparing normal cells. Initial research focussed on naturally-occurring non-pathogenic viruses, however, these studies were of limited success. Although tumour growth was observed to slow down and normal tissue was not damaged, there was no alteration in the course of the disease.
Recent studies have therefore focussed on engineering recombinant viruses that selectively target cancer cells. One example of this class of engineered viruses is adenoviruses that are mutated in the E1B region of the viral genome.
Adenoviral E1B and p53
One function of the mammalian tumour suppressor protein p53 is to mediate cell-cycle arrest and/or apoptosis in response to DNA damage or foreign DNA synthesis. Consequently, some viruses, such as adenovirus, encode proteins that inactivate p53 in infected cells to allow efficient viral replication. One of these proteins, the 55 kiloDalton protein from the E1B region of adenovirus (E1B-55K or E1B-496R), binds to p53 so causing a substantial loss of p53. This consequently prevents p53-mediated apoptosis of the infected cell. E1B-496R is therefore essential for adenoviral replication in cells containing functional p53.
Human tumour cells are frequently homozygous or heterozygous for mutated (e.g. substitution, deletion, frameshift mutated) p53 alleles, and lack p53 function necessary for normal control of the cell cycle (Hollstein et. al (1991) Science 253:49; Levine et al. (1991) Nature; 351(6326):453-6). Many neoplastic cells are therefore p53(−) either because they lack sufficient levels of p53 and/or because they express mutant forms of p53 which are incapable of substantial p53 function.
E1B Mutated Adenoviruses
Oncolytic adenoviruses have been engineered that take advantage of the difference in p53 functionality between neoplastic and normal cells. By mutating the E1B-496R protein to remove binding interactions with p53 or by making various deletions within the E1B locus (see, for example, U.S. Pat. No. 5,677,178), the resulting adenoviruses can replicate and ultimately lyse cancer cells that substantially lack p53 function, but not in cells that possess normal p53 function.
One example is ONYX-015 (originally named d11520 and also referred to as H101), a mutant adenovirus that does not express the E1B-496R protein (Heise et al. (1997) Nat. Med. 3 (6): 639-645). The virus contains a stop codon immediately following the translation start codon and also has a large deletion of the E1B-496R coding sequence. As a result this virus lacks the ability to bind and inactivate p53, and thus can only replicate efficiently in cells defective in p53 function, such as neoplastic cells and tumours. Unfortunately, E1B-496R carries out other functions in addition to binding and inactivating p53 (Eager et al. (2001) Cancer Gene Ther. 18 (5): 305-317). Consequently, the ONYX-015 virus is defective in cytoplasmic accumulation of the viral late mRNAs, host cell shut-off and translation of late mRNAs. Thus, the mutation in ONYX-015 compromises the ability of the mutant virus to reproduce itself in tumour cells. An additional problem is that large deletions destabilise the viral genome.
Additional examples are ONYX-051 and ONYX-053, mutant adenoviruses that contain point mutations (R240A and H260A, respectively) in the E1B-496R protein that prevent its binding to p53. These mutations enable the virus to replicate selectively in cells that are deficient in p53 function, without compromising the ability of the virus to replicate in these cells (Shen et al. (2001) J. Virol. 75 (9): 4297-4307 and U.S. Pat. No. 7,785,887).
However, there remains a great need for improved mutant viruses whose oncolytic ability has been enhanced and which are useful in the therapy of cancer.